docs(models): the record-splitting rule gets a findable home — note 3163 (#3128 rec 9)

Spike #3128's fourth question was whether a "when does a record type earn its
own table" rule was worth writing down. It turned out to already exist, in
full, in the RuleEmbedding docstring — the only written statement of a rule
Scribe applies to every record type, sitting where nobody would look for it.

Promoted to note 3163, with the three grounds (scoping different in kind,
machine-written at volume, edge-or-event-not-document), the worked cases
across the whole schema, and the bill: what `rules` had to re-import after
leaving `notes`, including the two cells it left empty on purpose.

The docstring stays put — it is where the decision was made — and now points
at the note.

src/scribe/models/embedding.py

@@ -51,7 +51,11 @@ class RuleEmbedding(Base):
     """One embedding vector per CHUNK of a rule (milestone 307, note 3026).
 
     A SIBLING of NoteEmbedding rather than a generalisation of it, decided
-    deliberately:
+    deliberately. The reasoning below turned out to be the only written
+    statement of a rule Scribe applies to every record type, so it is now also
+    NOTE 3163 — "When a record type earns its own table" — with the worked
+    cases and the cost of leaving `notes`. Read that before splitting a record
+    type off; this stays here because it is where the decision was made.
 
     - The embedding ROW could have been made polymorphic. The SEARCH could not.
       `semantic_search_notes` is a long function of Note-specific scoping —